There is a type of projectors which uses an image display device to modulate illumination light from a light source into image light, which is then magnified and projected on a screen through a projection lens. Since the projectors are getting smaller and smaller, the projection lens of the projectors is usually a difficult-to-manufacture aspherical lens so that the projection lens can fit into a small housing of the projectors.
In view of the manufacturability and the cost, the aspheric lenses are often made of plastic. Such plastic lenses are typically manufactured by injection molding, where molten plastic is injected into a mold. While the injection molding requires the use of a gate through which the molten plastic is injected, it is around the gate that the stream of the molten plastic changes drastically. As a result, the finished plastic lenses sometimes have defects, such as bubbles and striae, in the vicinity of a cut off mark of the gate (gate mark). When these defective plastic lenses are used as the projection lens, the bubbles and the striae may deteriorate the image quality.
As a countermeasure to this drawback, Japanese patent laid-open publication No. 2004-361620 discloses to locate the gate mark of the plastic lens outside an optical usage area. Here, the optical usage area means the area on the lens surface that the image light passes through.
Fortunately, resent improvements on the injection molding technique leads to reduce most of the above mentioned optical defects around the gate mark. On the other hand, however, it becomes recognized that the optical defect called a weld line is formed on the opposite side to the gate mark and impairs the optical performance of the plastic lens. It is also recognized that the weld line may extend to near the central portion of the lens. This symptom is especially prominent in a large diameter concave meniscus lens having the thicker peripheral portion than the central portion, such as the projection lens of the projector.
The weld line is a linear mark formed by the meeting of two streams of the molten plastic which is once separated after the injection through the gate in the mold. When making the concave meniscus lens having the lens thickness ratio of more than 2:1 as described above, the molten plastic firstly separates into two streams to proceed along the lens peripheral portion having low flow resistance, then meets at the opposite side to the gate, and flows into the lens central portion. Accordingly, in many cases, the weld line extends from the lens peripheral portion to the vicinity of the lens central portion. Technically, the weld lines can be reduces by controlling the molding conditions such as the selection of the plastic material, the plastic melt temperature, the mold temperature, the injection rate and so forth. It is, however, difficult to eliminate the weld lines completely while maintaining a certain level of optical performance and preventing the optical defects around the gate mark.